1. Field
The disclosed concept pertains generally to contacts for vacuum interrupters and, more particularly, to contact members for a vacuum envelope. The disclosed concept further pertains to vacuum interrupters including fixed and movable contacts.
2. Background Information
Vacuum interrupters include separable main contacts disposed within an insulated and hermetically sealed vacuum chamber. The vacuum chamber typically includes a number of sections of ceramics (e.g., a number of tubular ceramic portions) for electrical insulation capped by a number of end members (e.g., without limitation, metal components, such as metal end plates; end caps; seal cups) to form an envelope in which a vacuum may be drawn. The ceramic section is typically cylindrical; however, other suitable cross-sectional shapes may be used. Two end members are typically employed. Where there are multiple ceramic sections, an internal center shield is disposed between the ceramic sections.
Vacuum circuit interrupters (e.g., without limitation, vacuum circuit breakers; vacuum switches; load break switches) provide protection for electrical systems from electrical fault conditions such as current overloads, short circuits, and low level voltage conditions. Typically, vacuum circuit interrupters include a spring-powered or other suitable operating mechanism, which opens electrical contacts inside a number of vacuum interrupters to interrupt the current flowing through the conductors in an electrical system in response to abnormal conditions.
The main contacts of vacuum interrupters are electrically connected to an external circuit to be protected by the vacuum circuit interrupter by electrode stems, typically an elongated member made from high purity copper. Generally, one of the contacts is fixed relative to the vacuum chamber as well as to the external circuit. The fixed contact is mounted in the vacuum envelope on a first electrode extending through one end member. The other contact is movable relative to the vacuum envelope. The movable contact is mounted on a movable electrode axially slidable through the other end member. The movable contact is driven by the operating mechanism and the motion of the operating mechanism is transferred inside the vacuum envelope by a coupling that includes a sealed metallic bellows. The fixed and movable contacts form a pair of separable contacts which are opened and closed by movement of the movable electrode in response to the operating mechanism located outside of the vacuum envelope. The electrodes, end members, bellows, ceramic shell(s), and the internal shield, if any, are joined together to form the vacuum interrupter (VI) capable of maintaining a vacuum at a suitable level for an extended period of time.
With the wide acceptance of vacuum interruption technology in medium voltage switchgear, vacuum interrupters are being used in more and more demanding applications. One example is the ever increasing continuous current requirement. However, a high continuous current carrying capability is not easy to achieve, especially in an axial magnetic field (AMF) type VI, where the current is often forced into a relatively long circular path to generate the necessary axial magnetic field. A longer circular VI current path provides a stronger axial magnetic field and, hence, a better current interruption capability, although this increases the total resistance of the VI. For this reason, it is desirable to find ways to reduce the resistance of a VI without compromising its current interruption capability.
In known modern commercial vacuum interrupters, the mating surface on the arcing face of the movable contact and the fixed contact is two-dimensional (i.e., planar). See, for example, FIGS. 1 and 2. In these designs with a planar mating surface 2,4, the physical contact between the two opposing electrical contacts 6,8 and 10,12 often ends up taking place only at a limited number of discrete locations of the planar surfaces (e.g., a worst case scenario is three discrete locations), due to inevitable surface imperfections resulting from machining a fresh contact surface or roughening an existing contact surface from arc melting. As a result, the electrical resistance of the resulting joint, between the electrical contacts, can be significant.
In some older vacuum interrupters, it is known to provide movable and fixed contacts that mate in three dimensions, macroscopically (i.e., on at least two different surfaces of the contacts normal to the planar mating area, where the magnitude of the different surfaces are similar to the magnitude of the planar mating area). Examples are shown by U.S. Pat. Nos. 3,321,598 and 3,889,081.
There is room for improvement in vacuum interrupters.
There is further room for improvement in fixed and movable contacts of a vacuum interrupter.